[Q] Stupid mistake led to a null IMEI. Is it possible to recover the phone?

My sis asked me to sim unlock her s3 so I had to root it first. I downloaded the appropriate CF autoroot and went one to flashing it. However I accidentally chose the autoroot file for the s4 (as I was rooting it a few days ago). So I disconnected the phone (Odin got stuck) and then flahsed the correct autoroot.

The phone boots fine now but doesn't discover any network and the IMEI is indicated as missing.

I have the original IMEI written down, but no efs backup. The phone runs on android 4.3 and the bootloader is I9300XXUGNB5.

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